In resin encapsulated semiconductor devices (IC, LSI and transistor), Au wire with a diameter of 20-50 .mu.m is largely employed for providing an electrical connection between a semiconductor element and an external lead frame of a package. In bonding of the wire to the semiconductor element, there are two methods; one is a wedge bonding method and the other is a ball bonding method. These bonding methods are effected in such a way that one end of the Au wire or a ball (formed at an end of the Au wire through fusion thereof by means of arc discharge or hydrogen flames) is bonded to the semiconductor element directly or through a metallic deposited film by means of thermocompression bonding or ultrasonic bonding. On the other hand, the connection of the wire to the lead frame is effected by wedge bonding through a capillary.
However, instead of Au wire, the use of Al wire has been studied, since Au wire is expensive. Also in bonding of Al wire, wedge bonding by means of thermo-compression bonding or ultrasonic bonding has been investigated. However, the conventional Al wire, is used just as the wire is drawn, so that the wire material retains hardness due to the drawing operation. The inventors have found that in either the thermo-compression bonding or ultrasonic bonding of Al wire, a part of the wire near the bonding portion is heated and caused to soften, so that a local deformation takes place at the bonding portion in the wire connection from the semiconductor element to the lead frame, and that such a local deformation not only causes the wire to be locally reduced in diameter but also may result in breakage or the like.
Moreover, the inventors have discovered upon examination of the ball bonding of Al wire that since the conventional Al wire is wound on a reel (as it is being drawn as described above), the wire is elastically bent when a ball is formed at its end and the ball is eccentrically formed, and that since when the ball is formed, not only the end of the wire, but also a portion in the vincinity thereof are heated, the wire is partially subjected to annealing, and consequently, the wire is softened directly above the ball or locally constricted.
Furthermore, the inventors have found that the eccentricity of the ball may cause the bonding portion to project from a pad, bringing about short-circuiting with another bonding portion or damage to the circuit.
In addition, the inventors have discovered that if a constriction is produced directly above the ball or if the wire is caused to soften, the residual work set of the wire prevents the wire and the bonding portion from following a fine curve since the portion of the wire not annealed remains in a work-hardened condition; consequently, the wire is bonded in a bent shape, and this may cause breakage of wire. Particularly, in a dual in-line type IC or LSI having a difference in levels between the bonding surfaces of a semiconductor element and a lead frame, this may largely cause a short-circuit between the wire and the semiconductor element.
In case of ceramic packaged semiconductor devices, only a mechanical stress is applied to the bonding portion, since in the ceramic package the wire is not constricted by the package. On the other hand, in the resin encapsulated semiconductor devices a large mechanical stress is applied to the wire because it is rigidly molded with thermosetting resins such as epoxy resins, phenol resins, polyester resins, etc. The mechanical stress at the bonding portion on the semiconductor chip is maximum. Thereafter, it has been found that in resin encapsulated semiconductor devices, the wire materials play a very important role in the production of satisfactory ball-bonded connections.